Friday, July 24, 2015

Showers And Clouds Suppressed Storms Yesterday - Repeat Today?

First, a review of yesterday. As is typical, the morning showers put a severe damper on storm development in eastern Pima County for the remainder of the day. The composite radar chart from 5:00 pm MST yesterday afternoon (above) shows weak echo across the entire local region from Nogales to Oracle, with thunderstorms far removed to the east and west of Tucson. Plot of detected CG flashes (from Vaisala and Atmo below, through 11 pm last evening) shows the same pattern, with a large void of thunderstorm activity reflecting the path of the morning showers. 

Not surprisingly, rainfall did not live up to expectations, given the extremely moist atmosphere. From 10:00 am (after the showers had mostly exited to the northwest) through 5:00 pm only 17 ALERT sites measured rainfall amounts that were very light (exception was 0.59" at Dan Saddle in the Catalinas). Then for the period from 5:00 pm through 5:00 am this morning 31 sites had rainfall from cloud-debris showers moving in from the south. The nighttime amounts were very light, with the heaviest outlier being 0.28" on Redington Pass. Here at the house we had 0.08" from the morning showers and then 0.03" as the evening showers passed by. All-in-all, quite a dismal day for late July.

This morning has arrived with some sunshine. The visible satellite image - above for 6:00 am MST - shows a broad swath of cloudiness that extends across all of Sonora, eastern Arizona and into Colorado, but with large breaks over southeastern Arizona. Presently skies are mostly clear over the Catalinas as per campus webcam view below. We can start the day with better hopes than yesterday.

This morning's skewT plot of the 12 UTC TWC sounding data (above from SPC) remains quite similar to those of the past couple of mornings - very wet with CAPE available given heating, but with a hostile wind profile for the metro area. The light/variable winds in the lower troposphere now are at 700 mb and below, with light southerly steering winds for thunderstorms. Matt Bunker's PW analysis for the 12 UTC soundings this morning (below - links are available in an earlier post) shows the TWC sounding to be by far the wettest in the Southwest.

Possible problems for this afternoon, even with sunshine, include: the heating will build a new boundary layer up to about 750 - 800 mb but this will have some CIN above that would need outflow help at low elevations; upper-level wind profile is quite negative and would again stream anvil clouds from south over the metro area (indeed, several of the WRF forecasts for the afternoon show this happening); and some of the WRF forecasts begin drying out our low-levels from the northwest this afternoon.

I have looked at the WRF runs from both 00 and 06 UTC last evening - I can not really find a variant that does not seem to have some problems. Some appear too dry, some too cloudy, some too stable, and etc., so I'll wait for Mike's discussion of the new 12 UTC forecast runs. However, none of the four versions forecast thunderstorms at low elevations of eastern Pima County. The 00 UTC WRF-NAM forecast of composite radar echoes for 4:00 pm this afternoon (below) is as active as it gets in that forecast. Other versions have more storm activity on the mountains right around Tucson. I tend to feel that these early forecasts from WRF are a bit conservative, and that we'll see more thunderstorm activity nearby this afternoon - which is probably a reasonable guess, given yesterday's total lack of nearby thunderstorms.

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